Device for centering a rotational unit

ABSTRACT

A device for centering a rotational unit on a rotatable hub for rotation of the unit together with the hub. A centering surface which extends around the circumference of the hub extends essentially from a mounting edge for the rotational unit to a stop surface which determines the axial position of the rotational unit. The rotational unit presents in a corresponding way a centering surface which in its one end transcends into a stop surface, which determines the radial position and which is intended for contact with the stop surface of the hub. One of the centering surfaces is divided into a first centering section with a certain larger clearance to the second centering surface and a second centering section with a certain smaller clearance to the second centering surface. The first centering section extends across the main part of the axial distance from the stop surface of the hub to its mounting edge. The second centering section extends across a minor section of the axial distance from the stop surface to the mounting edge of the hub.

TECHNICAL FIELD

The present invention relates to a device for centering a rotationalunit, on a rotational hub. More specifically, the invention relates to adevice wherein both the hub and the rotational unit have mountingsurfaces for guiding the position of the rotational unit both axiallyand radially.

BACKGROUND OF THE INVENTION

In many applications it is of great importance that the rotating unitsare properly centered in relation to their axes of rotation. Suchrotational units, which are mounted on a rotary hub via mountingsurfaces, normally present centering surfaces, shaped as cylindermantles, which are centered against corresponding centering surfaces,shaped as cylinders mantles, on the hub. Since, for production reasons,a certain tolerance range for the centering surfaces of both therotational unit and the hub exists, the centering surfaces must bedimensioned in such a way that a certain minimum permitted clearance isassured. In certain cases, the clearance may become so large that, to agreat extent, the rotational unit becomes excentrically mounted on thehub and that there is an increased risk for it becoming stuck due torust, which makes its removal more difficult.

Applications in which eccentrically mounted rotational units may involvefunctional problems are for example brake drums for motor vehicles. Aneccentrically mounted brake drum results in a varying braking torque,which in turn leads to vibrations in the steering wheel and/or thevehicle passenger compartment, which normally is perceived as veryannoying by the driver.

SUMMARY OF THE INVENTION

The object of the present invention is to eliminate the above-mentionedproblems regarding rotational units by means of a satisfactorycentering, while maintaining favorable qualities for mounting and areduced risk for it becoming stuck due to rust (fitting rust).

Said object is achieved by means of a device according to the presentinvention, the characterizing features of which will become apparentfrom the subsequent disclosure and within claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in greater detail with anembodiment, with reference to the annexed drawings, in which:

FIG. 1 shows an axial cross-section of a rotational unit in the form ofa brake drum mounted on a hub, according to the present invention,

FIG. 2 shows, on a larger scale, a section of the brake drum and thehub,

FIG. 3 shows, on a further enlarged scale, a section of the brake drumand the hub clearly showing their mutual mounting surfaces,

FIG. 4 shows the brake drum and the hub during a first mounting stage,and

FIG. 5 shows an end view of the brake drum.

PREFERRED EMBODIMENTS

The chosen example shows a rotational unit in the form of a brake drum 1which is mounted on a hub 2 which is rotatable around a geometric axis 3of rotation. The hub may be fixed to a (not shown) drive shaft of amotor vehicle, the brake drum 1 being adapted to be rotatably fixed tothe hub and thus adapted to rotate together therewith, and thus alsotogether with the drive shaft. The brake drum is normally arranged onthe inside of a vehicle wheel and adapted to rotate together with thewheel. The brake drum forms part of a conventional brake system with(not shown) brake shoes which are carried by the vehicle so that theyessentially do not rotate with the brake drum but are adapted to brakethe vehicle by frictional contact against the cylindrical brake surface4 of the brake drum, the centricity of which in relation to the axis 3of rotation is of great importance for the braking function and comfortin the vehicle. The brake drum is a separate rotational unit with a rearsection 5 which extends essentially in a radial plane and which presentsmounting surfaces 6, 7 intended for cooperation with correspondingmounting surfaces 8, 9 on the hub 2.

FIG. 2 shows a slightly enlarged partial view of FIG. 1, which is chosenso that it is apparent which section of the brake drum and the hub formsthe mounting surfaces 6-9. The mounting surfaces form guiding surfaceswhich secure the position of the brake drum, partly for centering, i.e.a guiding in a radial direction, and also for guiding or securing theposition in an axial direction. Also, the axis of symmetry and the axis3 of rotation of the drum should coincide as far as possible, i.e. theyshould not present any relative angle. The guiding in the axialdirection is assured by the first mounting surface 8 of the hub 2, whichextends in a radial plane and forms a stop surface for the correspondingmounting surface 6 of the drum 1. The guiding in the radial direction isassured by the centering surface 9 of the hub, which is shapedessentially as a cylinder mantle and which is radially outwardly facing(convex), and also by the centering surface 7 of the drum which is alsoshaped essentially as a cylinder mantle and is radially inwardly facing(concave) towards the axis 3 of rotation.

The shaping of the mounting surfaces appears in greater detail in FIGS.3 and 5. From these figures it is apparent that the cylindricalcentering surface 9 on the hub extends, as seen in the axial direction,essentially between the stop surface 8 of the hub and the free edge 10of the hub which presents a sloping section 11 which is shaped as themantle of a cone in order to facilitate the mounting of the drum 1. Thecentering surface 7 of the drum, which surface is shaped as a cylindermantle, is divided according to the invention into two centeringsections 12, 13, of which a first centering section 12 is located nextto the stop surface 6 of the drum and extends over the main part of theentire centering surface of the drum, i.e. the surface which cooperateswith the centering surface 9 of the hub. The first centering section 12,which is shaped essentially as a cylinder mantle, is arranged at aradial distance from the axis 3 of rotation of the drum such that thiscentering section presents a relatively larger clearance with respect tothe centering surface 9 of the hub 2. The second centering section 13 ispositioned beyond the first centering section, as seen from the stopsurface 6, and is positioned in relation to the first centering section13 at a somewhat smaller radial distance from the axis 3 of rotation ofthe drum.

As is apparent from FIG. 5 and according to a preferred embodiment, thecentering section 13 is divided into four partial surfaces 17, 18, 19and 20, each of which only occupying 10°-15° of the mantle surface of anentire cylinder. According to the invention, the second centeringsection occupies a shorter distance "a" of the active width "b" of thecentering surfaces than the first centering section, for example lessthan a fifth, more precisely for example a tenth of the total width "b"of the centering surface 9 of the hub. In absolute dimensions, forexample the width "a" of the second centering section 13 may beapproximately 1 mm, whereas the width b of the centering surface 9 ofthe hub may be approximately 10 mm. The clearance between the drum 1 andthe hub 2 over the first centering section 12 may for example be Δ₁=0.056→+0.0218 mm, whereas the corresponding clearance over said secondcentering section 13 may be for example Δ₂ =0.08 →+0.06 mm. With theproposed clearance, it has been verified that the radial distorsion hasbeen reduced by approximately 0.05 to approximately 0.1 mm. Due to thefact that the width "a" is chosen so small, the clearance may thus evenbe negative before the mounting, thereby involving a plastic deformationof the active centering surfaces. During mounting, a deformation of saidfirst and/or second centering section 12, 13 occurs, whereby essentiallya zero clearance is obtained, i.e. a full contact between the centeringsections 12, 13. The section which is deformed, or deformed the most,depends mainly on the difference in hardness between the centeringsections. By intermittently dividing the centering section 13, as isshown in FIG. 5, this deformation is facilitated.

Between the two centering sections 12, 13 there is a transition surfacein the form of a sloping surface 14, which has the shape of a conemantle and which, due to its shape provides a successive transitionbetween the centering sections and thereby also an easier mounting.

With the design of the mounting surfaces 6, 7, 8, 9 of the brake drum 1and the hub 2, respectively, as described above, a considerably improvedcentering of the brake drum 1 in relation to the axis 3 of rotation ofthe hub 2 is thus obtained, without making the mounting of the brakedrum more difficult. The manner in which the brake drum is mounted onthe hub will now be described with reference mainly to FIG. 4. The brakedrum 1 is mounted on the hub 1 with an essentially axial lateralmovement in the direction of the arrow 15, with assistance firstly fromboth the sloping edge surface 16 of the drum and the sloping edgesurface 11 of the hub, in order to achieve a guiding towards thecentering surfaces. Thereafter, the centering section 12 provides afirst approximate centering of the drum brake in a first mounting stage,which is shown in FIG. 4. In practice, as is also shown in FIG. 4, thebrake drum 1 is not maintained in an entirely correct centered manner inrelation to the hub 2, but the first centering section 12 will, at leastalong some section of its circumference, be in contact with thecentering surface 9 of the hub, thus providing an approximate centeringof the drum during the main part of the mounting motion of the drum onthe hub. In a second and final mounting stage which ends with thestationary mounted condition of the drum which is shown in FIG. 3, afinal centering of the drum 1 in relation to the hub 2 is carried out bymounting the second centering section 13 with its smaller clearance onthe front section of the centering surface 9 of the hub. In practice, inmost cases the drum is mounted by pressing, due to the narrow or evennegative tolerances. The press mounting is preferably carried out bytightening screws in screw joints which extend through the drum and themounting surfaces 6, 8. With negative tolerances, the above-mentioneddeformation takes place in the edge and centering surfaces 9, 11, 13,14, which are pressed towards each other. The guiding to this positionis assured by means of the sloping surface 14 which thus forms atransition between the first and the second centering section. Arotationally fixed fastening of the drum to the hub is usually assuredby means of the above-mentioned screw joint. For production reasons, thescrew joints themselves can not satisfy the demand for centering of thedrum.

The invention is not limited to the embodiments mentioned above andshown in the drawings, but may be varied within the scope of theappended claims. For example, the rotational unit 1 may be somethingelse besides a brake drum. Furthermore, the sloping 14 may have another,for example a rounded, cross-sectional line. The surfaces which areshaped as cylinder mantles need not be shaped continously as cylindermantles, as seen in a radial plane, but may be formed as splines.Furthermore, the mounting surfaces 6, 8 do not have to be continous butmay be profiled in order to provide a rotationally fixed form engagementbetween the drum and the hub. The sloping and thus also the smallercentering section may be arranged in the radially inner part, in theshown example the hub, and is thereby placed closest to the stop surfaceof the hub, that is, inside of the larger centering section with agreater clearance. The hub is assumed to be the unit which iscentrically rotatable around the axis 3 of rotation. However, this doesnot have to be the radially inner part with a radially outwardly facingcentering surface, but may instead be the radially outer unit with aradially inwardly facing centering surface, whereas the rotational unitconstitutes the radially inner unit with a radially outwardly facingcentering surface. The second centering section 13 may consist of threeor more smaller partial surfaces, alternatively an entire cylindricalsurface or two larger partial surfaces. In embodiments with onecentering section 13 which consists of smaller individual partialsurfaces, the distance "a" may be chosen greater than in the case withone single surface. The active area, the tendency to be deformed and theclearance are, together with the coefficient of friction of thesurfaces, important parameters which influence the force needed for themounting.

We claim:
 1. A centering device comprising:(a) a rotatable hub,having(1) a radially outwardly facing circumferential centering surfacehaving first and second ends; (2) a hub stop surface extending from saidsecond end of said outwardly facing centering surface; and (3) amounting edge adjacent said first end of said outwardly facing centeringsurface; and (b) a rotational unit mountable on said rotatable hub forrotation therewith, having(1) a radially inwardly facing circumferentialcentering surface for mounting on said radially outwardly facingcentering surface of said rotatable hub and thereby determining arelative radial position between said rotatable hub and said rotationalunit, said inwardly facing circumferential centering surface havingfirst and second ends; and (2) a rotational unit stop surface adjacentsaid second end of said inwardly facing centering surface for contactingsaid hub stop surface and thereby determining a relative axial positionbetween said rotatable hub and said rotational unit; (c) wherein, in amounted position, said rotational unit stop surface contacts said hubstop surface, and one of said centering surfaces further comprises(1) afirst centering section extending a first portion of an axial distancefrom said hub stop surface to said mounting edge, said first centeringsection having a first clearance with respect to the other of saidcentering surfaces; and (2) a second centering section extending asecond portion of said axial distance from said hub stop surface to saidmounting edge, said second centering section having a second clearancewith respect to said other of said centering surfaces; said secondclearance being smaller than said first clearance; said second portionof said axial distance from said hub stop surface to said mounting edgebeing smaller than said first portion; and (d) said other of saidcentering surfaces comprising a single, constant diameter cylindricalmantle; (e) wherein, as said rotational unit is mounted on said hub,said other of said centering surfaces encounters said first centeringsection before encountering said second centering section.
 2. Acentering device as claimed in claim 1, wherein said centering surfacesare shaped substantially as cylinder mantles.
 3. A centering device asclaimed in claim 1, further comprising a transition between thecentering sections having a sloping surface.
 4. A centering device asclaimed in claim 1, wherein said second portion of said axial distancebetween said hub stop surface and said mounting edge is less than afifth of a total axial distance between said hub stop surface and saidmounting edge.
 5. A centering device as claimed in claim 1, wherein saidsecond centering section is divided into several partial surfaces.
 6. Acentering device as claimed in claim 5, wherein the partial surfacesoccupy a minor section of a total circumference of said second centeringsection.
 7. A centering device as claimed in claim 6, wherein thepartial surfaces are at least three in number.
 8. A centering device asclaimed in claim 6, wherein the partial surfaces are symmetricallydistributed around said circumference.
 9. A centering device as claimedin claim 6, wherein each of the partial surfaces occupies approximately10°-15° of said circumference.
 10. A centering device as claimed inclaim 1, wherein said second clearance is negative before the mounting,and at least one of said first and second centering surfaces is adaptedto be deformed essentially to a zero clearance during the mounting. 11.A centering device comprising:(a) a rotatable hub, having(1) a radiallyoutwardly facing circumferential centering surface having first andsecond ends; (2) a hub stop surface extending from said second end ofsaid outwardly facing centering surface; and (3) a mounting edgeadjacent said first end of said outwardly facing centering surface; and(b) a rotational unit mountable on said rotatable hub for rotationtherewith, having(1) a radially inwardly facing circumferentialcentering surface for mounting on said radially outwardly facingcentering surface of said rotatable hub and thereby determining arelative radial position between said rotatable hub and said rotationalunit, said inwardly facing circumferential centering surface havingfirst and second ends; and (2) a rotational unit stop surface adjacentsaid second end of said inwardly facing centering surface for contactingsaid hub stop surface and thereby determining a relative axial positionbetween said rotatable hub and said rotational unit; (c) wherein, in amounted position, one of said centering surfaces further comprises(1) afirst centering section extending a first portion of an axial distancefrom said hub stop surface to said mounting edge, said first centeringsection having a first clearance with respect to the other of saidcentering surfaces; and (2) a second centering section divided intoseveral partial surfaces occupying a minor section of a totalcircumference of said second centering section and extending a secondportion of said axial distance from said hub stop surface to saidmounting edge, said second centering section having a second clearancewith respect to said other of said centering surfaces; said secondclearance being smaller than said first clearance; said second portionof said axial distance from said hub stop surface to said mounting edgebeing smaller than said first portion; and (d) wherein, as saidrotational unit is mounted on said hub, said other of said centeringsurfaces encounters said first centering section before encounteringsaid second centering section.
 12. A centering device as claimed inclaim 11, further comprising a transition between the centering sectionshaving a sloping surface.
 13. A centering device as claimed in claim 11,wherein said second portion of said axial distance between said hub stopsurface and said mounting edge is less than a fifth of a total axialdistance between said hub stop surface and said mounting edge.
 14. Acentering device as claimed in claim 11, wherein the partial surfacesare at least three in number.
 15. A centering device as claimed in claim11, wherein the partial surfaces are symmetrically distributed aroundsaid circumference.
 16. A centering device as claimed in claim 11,wherein each of the partial surfaces occupies approximately 10°-15° ofsaid circumference.
 17. A centering device as claimed in claim 11,wherein said second clearance is negative before the mounting, and atleast one of said first and second centering surfaces is adapted to bedeformed essentially to a zero clearance during the mounting.